1. Technical Field
The present invention relates to an aqueous ink composition and a printing process by an ink jet system using the composition.
2. Related Art
A printing process by an ink jet recording system is performed by letting ink droplets fly and attach to a recording medium such as paper. Because of recent innovative progress in ink jet recording technology, the printing process employing the ink jet recording system has been also used in the field of highly fine image recording (image printing), which was ever achieved only by photograph or offset printing. Accordingly, there are demands for high-quality printed matters formed not only on plain paper and exclusive paper for ink jet recording (e.g., matte paper and gloss paper), which are usually used, but also on non-ink-absorbing or low-ink-absorbing recording media such as printing paper, synthetic paper, and films.
Many ink compositions that give high-quality images on plain paper or exclusive paper for ink jet recording have been proposed. Specifically proposed is an ink composition composed of at least a pigment coated with a water-insoluble polymer, a specific glycol ether, and a 1,2-alkyldiol (see JP-A-2009-235155).
Recently, instead of solvent-based pigment inks, which have been used for film recording media such as polyvinyl chloride materials, that is, non-ink-absorbing recording media, aqueous inks have been used from the viewpoints of safety and protection of the environment. As an example using such an aqueous ink, specifically proposed is a method of printing on a hydrophobic surface using an ink containing water, a glycol solvent, an insoluble coloring agent, a polymer dispersing agent, a silicone surfactant, a fluorinated surfactant, a water-insoluble graft copolymer binder, and N-methylpyrrolidone (see JP-A-2000-44858). Furthermore, a polymer colloid-containing aqueous ink jet ink composed of an aqueous liquid vehicle containing a volatile co-solvent having a boiling point of 285° C. or lower, acid-functionalized polymer colloid particles, and a pigment coloring agent for printing on non-porous base materials is proposed (see JP-A-2005-220352).
However, the printing quality of the image formed on a recording medium by the proposed ink or printing process using such an ink is not sufficiently excellent. Accordingly, there have been demands for an ink composition that has been further improved in the characteristics described above and a printing process using the ink composition. Specifically, a phenomenon of color mixture between different colors or spreading of ink to a non-desired area (this phenomenon is referred to as “bleeding”) occurs, or a phenomenon that a solid image has thick areas and thin areas of ink (this phenomenon is referred to as “uneven ink density”) occurs, thus, in known ink compositions and printing processes using the known ink compositions, desired printing quality has not been obtained. Furthermore, in aqueous inks that have been proposed, the abrasion resistance of printed matters formed on non-ink-absorbing or low-ink-absorbing recording media is not sufficiently excellent. Accordingly, there has been a demand for a printing process by an ink jet recording system to obtain printed matters that are more excellent in abrasion resistance.